Given a binary tree, you have to return the size of it. Size of binary tree is defined as number of nodes in it.
Examples:
Input:
1
/ \
2 3
Output: 3
Explanation: There are three nodes in given binary tree.
Input:
10
/ \
5 9
\ / \
1 3 6
Output: 6
Explanation: There are six nodes in given binary tree.
Input:
1
Output: 1
Explanation: There is one nodes in given binary tree.
Constraints:
1 <= number of nodes <= 105
1 <= node->data <= 105
